major overhaul for thread-safety - many global variables and static

major overhaul for thread-safety - many global variables and static
buffers eliminated, this causes MANY changes, such as va() taking a
buffer, prog variable has to be passed as parameter to every PRVM
function, and many other edits of that nature.
realtime is now the only linearly increasing time stamp in the engine,
Sys_DoubleTime replaced by Sys_DirtyTime which requires sanity checks in
every use, but ensures thread-safe behavior
added -Wmissing-prototypes to makefile so that no one will ever put an
extern function prototype in a .c file again, this was necessary to
clean up crashes where prototype parameters differed.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@11463 d7cf8633-e32d-0410-b094-e92efae38249